When performing weld operations on critical components it is often necessary to immerse the part being welded in an inert atmosphere. The inert atmosphere protects the weld from contamination, allowing the creation of extremely clean and defect free fusion bonds. Argon is commonly used as the inert cover gas, since it is heavier than air. It is common practice to place the object being welded in a "purge box", which is a sealed chamber open at the top. Argon is metered into the bottom of the box, and flows out the top. The object to be welded is placed in the box, and is completely immersed in the argon. The open top allows the argon to displace the air by pushing it out from the bottom, and also allows access by the welder.
A type of purge box is used during CO.sub.2 laser welding of turbine airfoils as disclosed in U.S. Pat. No. 5,554,837. It is similar to the type described above, except that the "box" is a clear quartz cylinder. The quartz cylinder allows the operator to clearly see the weld process from safety outside the laser enclosure. When performing laser welding with the use of induction coils to preheat the part, the quartz, which is transparent to ultraviolet radiation, allows the use of ultraviolet sensors to control the induction coil preheat temperature.
A problem arises, however, when using argon purge boxes in general and quartz cylinders specifically during laser welding. For any purge box to be effective the part to be welded must be significantly below the edges of the purge chamber. That way the part remains completely immersed in the inert argon atmosphere. It can be difficult to position a part to be welded deep enough in the argon for effective shielding due to the access that is required by the welder; or when laser welding, by interference between the induction coils, powder feed nozzles, and the edges of the actual purge box (or cylinder).
The most common method of solving this problem is to increase the flow of argon in the purge box, and/or add additional argon flow through the weld nozzles. This solution is sufficient in many cases, but not all. significant difficulties arise when performing preheated induction welding. Elevating the part to temperatures in excess of 1200.degree. F. prior to weld creates strong updrafts of the protective cover gas. The heated, rising argon creates turbulence, which can mix air in with the argon and contaminate the weld. The situation is even worse when the part cannot be located much below the edge of the purge box.